During an appearance on Jimmy Kimmel Live Wednesday evening, Letterman, 73, told the story of when he had Philbin on his late-night CBS show the night before he was retiring from hosting Live! With Regis and Kelly in 2011.
One person from the Late Show With David Letterman suggested they give Philbin a Rascal Scooter as a retirement gift as "a joke about how old and feeble" he was, Letterman recalled.
"I said, 'No, no. Let's not give Regis a Rascal Scooter," Letterman continued. "Let's give him an actual scooter, like a Vespa. Everybody knows how to ride a motor scooter."
"So that's what we decided to do — 'Regis, we're so excited. We're sorry you're retiring, but to commemorate the evening and your last show tomorrow, we're giving you this beautiful, brand-new Vespa scooter,' " he said.
Kimmel then played a clip of what happened next. Right after getting on the scooter and waving goodbye, Philbin took a fall off the Vespa. "He could have been killed. He actually could have been killed," the My Next Guest Needs No Introduction host said.
"The last night before he retires he comes over, and I kill him," Letterman said with a laugh. "Nobody checked him out on it, because the assumption was, A, anybody can ride a scooter and B, certainly Regis will ride a scooter."
Luckily for Letterman, Philbin got right back up after his fall, declaring, "I made it!," and even gave the scooter another try later on in the show.
RELATED VIDEO: Jimmy Kimmel Honors Regis Philbin in Touching Who Wants to Be a Millionaire Tribute
In addition to retelling the hilarious story, Letterman reflected on what Philbin meant to him, calling him a "tremendous individual" with a "great heart."
Philbin, who died in July at the age of 88, represented "the last connection to a show business that I grew up watching back in Indiana," Letterman told Kimmel.
"When Regis left us, to me, it seemed like, 'OK, that chapter is closed.' That made me very sad," he added. "But I'm telling you, this guy, if somebody said, 'Let's pick somebody to drive across country with,' if it couldn't be Regis, then I would hitchhike."
In July, Letterman shared a heartfelt statement on Twitter in tribute to Philbin. "Regis is in the same category as [Johnny] Carson," he wrote. "Superlative. He was on our show a million times, always the best guest we ever had, charming, lovable and could take a punch. When he retired I lost interest in television. I love him."